Ab initio band-structure studies of beryllium and beryllium-hydrogen ultrathin films

Global minimum beryllium hydride sheet with novel negative Poisson's ratio: first-principles calculations

As one of the most prominent metal-hydrides, beryllium hydride has received much attention over the past several decades, since 1978, and is considered as an important hydrogen storage material. By reducing the dimensionality from 3 to 2, the beryllium hydride monolayer is isoelectronic with graphene; thus the existence of its two-dimensional (2D) form is theoretically feasible and experimentally expected. However, little is known about its 2D form. In this work, by a global minimum search with the particle swarm optimization method via density functional theory computations, we predicted two new stable structures for the beryllium hydride sheets, named α-BeH2 and β-BeH2 monolayers. Both structures have more favorable thermodynamic stability than the recently reported planar square form (Nanoscale, 2017, 9, 8740), due to the forming of multicenter delocalized Be-H bonds. Utilizing the recently developed SSAdNDP method, we revealed that three-center-two-electron (3c-2e) delocalized Be-H bonds are formed in the α-BeH2 monolayer, while for the β-BeH2 monolayer, novel four-center-two-electron (4c-2e) delocalized bonds are observed in the 2D system for the first time. These unique multicenter chemical bonds endow both α- and β-BeH2 with high structural stabilities, which are further confirmed by the absence of imaginary modes in their phonon spectra, the favorable formation energies comparable to bulk and cluster beryllium hydride, and the high mechanical strength. These results indicate the potential for experimental synthesis. Furthermore, both α- and β-BeH2 are wide-bandgap semiconductors, in which the α-BeH2 has unusual mechanical properties with a negative Poisson's ratio of -0.19. If synthesized, it would attract interest both in experiment and theory, and be a new member of the 2D family isoelectronic with graphene.

Dirac node lines in two-dimensional Lieb lattices

As a new type of quantum matter, Dirac node line (DNL) semimetals are currently attracting widespread interest in condensed matter physics and materials science. The DNL, featured by a closed line consisting of linear band crossings in the momentum space, was mostly predicted in three-dimensional materials. Here, we propose a tight-binding (TB) model of p_z + p_x,y or p_z + s orbitals defined on the two-dimensional (2D) Lieb lattice for the 2D version of DNL semimetals. The DNL states in these models are caused by the inversion of the bands with different symmetries and thus robust against spin-orbit interaction. By means of first-principles calculations, we demonstrate two candidate materials: Be₂C and BeH₂ monolayers, which have Fermi circles centred at Γ(0,0) and K(1/2,1/2) points, respectively. Their Fermi velocities are higher than that in graphene. The non-zero Z₂ topological invariant accompanied by the edge states is revealed in these materials. This work opens an avenue for the design of 2D DNL semimetals.